Methods and apparatuses of prior art are described for instance in publications FI-74496, DE-3 639 200 and EP-279 022. The methods and apparatuses presented in these publications include a separate wash space which is in connection with the pulping space and which is relatively small in volume compared to that of the pulping space. Therefore, these known methods and apparatuses require the use of separate attachments for connection to the actual pulping space. Handling of the rejected portion is particularly difficult and application of the methods requires a substantial amount of wash water. The capacity of the known methods is low and some of them involve batch operation.